Scanners for electronically forming an image of an original are known. Typically, the captured image provided by a scanner is a pixel data array that is stored in memory in a digital format. A distortion-free image requires a faithful mapping of the original image to the pixel data array. Scanners typically include at least one means for imposing a mechanical constraint during the image capture process in order to maximize the likelihood of faithful mapping. The four types of scanners known in the art are drum scanners, flatbed scanners, two-dimensional array scanners and hand scanners.
Drum scanners attach the original to the surface of a cylindrical drum that rotates at a substantially fixed velocity. During the rotation of the drum, an image sensor is moved in a direction parallel to the rotational axis of the drum. The combination of the linear displacement of the image sensor and the rotation of the original on the drum allows the entire original to be scanned. At any moment during the imaging process, the current position within the pixel data array relative to the original can be determined by measuring the angular position of the drum and the translational position of the sensor. The position of the pixel data array with respect to the original is fixed as long as the original is properly attached to the drum, the drum rotation is properly controlled, and the sensor is properly controlled in its displacement along the linear path.
Flatbed scanners include a linear array sensor that is moved relative to the original along an axis that is perpendicular to the axis of the array. Thus, the position of the sensor in one dimension may be known by tracking the relative movement of the sensor. The position of the sensor in the perpendicular direction is implicitly fixed by addressing a particular array element at which intensity is to be measured. In one embodiment of the flatbed scanner, the original is placed on a transparent platen and the sensor, along with an image illumination source, is placed on a side of the platen opposite to the original. As long as the original is not moved relative to the platen, the pixel data array will be fixed with respect to the image to be captured. In another embodiment, the original is moved, rather than the sensor. This second embodiment is typical of facsimile machines. Precision paper transports provide a high degree of positional accuracy during the image-capture process.
Advantages of the drum and flatbed scanners include the ability to accommodate documents at least as large as A4, or 8.5".times.11" paper. Moreover, some of these scanners can handle A1 paper in a single setup. However, the scanners are not generally portable, since they require a host computer for control, data storage and image manipulation.
Two-dimensional array scanners may be used in the absence of mechanical encoding constraints, and require only that the array and the original be held motionless during an exposure period. A two-dimensional array of photosensitive elements directly accomplishes the mapping of the image of the original into a pixel data array. However, because a single 300 dpi mapping of an 8.5".times.11" original requires an image sensor having an array of 2500.times.3300 elements, i.e. 8.25 million pixels, these scanners are cost-prohibitive in most applications.
Conventional hand scanners require a user to move a linear array of electrooptical sensor elements over an original. The movement is by hand manipulation. Array-position information is determined using methods such as those employed in operation of a computer "mouse." As a linear sensor array is moved, the rotation of wheels, balls or rollers that are in contact with the original is sensed, and the position information is determined from the mechanical details of the rotation. In general, the surface of the mechanical element in contact with the original has a high coefficient of friction, e.g. rubber, so as to resist slip and skid. A cylindrical roller or two wheels connected by a rigid axle may be used to enforce a single translational degree of freedom during the scanning process. A straight-edge or other fixture is often used to fix the scan direction with respect to the original and to further enforce the translational constraint provided by the pair of wheels or the roller. Nevertheless, the position encoder approach is one that is often susceptible to slips and skips, so that the pixel data array loses its correspondence with the image on the original.
Hand scanners are typically connected directly to a separate computer for image data storage, processing, and use. Data rates from the image sensor tend to limit the scanning speed. The scanners provide feedback to the user, typically by means of green or red light emitting diodes, to maintain the appropriate speed for the desired image resolution. Some hand scanners use electromagnetic brakes to prevent the user from dragging the scanner over the image too rapidly, with the mechanical resistance increasing with increases in scanning speed.
Hand scanners utilize relatively small imaging arrays and generally cannot handle larger than A6 documents in a single pass. This requires stitching algorithms to join together multiple swaths of a larger document. Swath stitching may be done in a separate operation by a separate computer. Scanning a multi-page business document or report with a hand scanner is a tedious process that often yields low-quality results. Techniques for stitching image swaths are known in the scanning art. These techniques typically require a pair of complete image swaths and produce a single, global transformation which brings the two swaths into registration. Improved techniques are taught in the International Patent Application Publication No. WO 96/27257.
In forming a scanned electronic image it is necessary to acquire navigation information that is acquired along with image data. A hand scanner in which navigation information is obtained without compromise to ease of use of the scanner is described in U.S. Pat. No. 5,578,813. The navigation information is acquired by means of at least one navigation sensor that detects inherent structure-related properties of an original being scanned. Movement of an image sensor along the original is tracked by monitoring variations of the inherent structure-related properties as the image sensor is moved relative to the original. The inherent structure-related properties that are monitored are inherent structural features, such as paper fibers, or other constituents of the original. Navigation can alternatively be speckle-based, wherein movement of the image sensor along the original is tracked by monitoring variations of speckle patterns produced using coherent illumination for acquiring the navigation information.
"Inherent structure-related properties" are properties of the original that are attributable to factors that are independent of forming image data and/or of systematic registration data on the original. The navigation information may be formed by generating a position signal that is responsive to detection of inherent structure-related properties, such as a position signal of speckle information or a position signal that permits tracking of individual inherent structural features. "Inherent structural features" are those features of an original that are characteristic of processes of forming the original and are independent of forming image data and/or systematic registration data on the original. For example, if the original recorded media is a paper product, the inherent structural features of interest may be paper fibers. As another example, navigation of the image sensor across a glossy original or an overhead transparency film may be determined by tracking surface texture variations that affect specular fields. Typically, the inherent structural features are microscopic, e.g. between 10 and 40 .mu.m, features of surface texture.
The present invention is directly applicable to hand scanners as taught in International Patent Application Publication No. WO 96/27257 and U.S. Pat. No. 5,578,813, both of which applications are incorporated herein by reference to the extent that this is permissible by national law.
As previously noted, some type of fixture is typically used with a hand scanner. In the absence of a fixture (as is the case for most embodiments in U.S. Pat. No. 5,578,813), there is a tendency to impose some rotation as the hand scanner is moved across an original. If the user's elbow is resting on a flat surface during movement of the scanner, the rotation is likely to have a radius defined by the distance between the scanner and the user's elbow. As a consequence, the scanned electronic image will be distorted. Other curvilinear movements during a swath of the scanner will also create distortions.
The present invention aims to provide a scanning device and method that produces high quality distortion free reproduction of an original even in the presence of distorted scanning paths. This allows the user greater freedom to scan a document by hand and allows the production of a composite image from the continuous scanning of overlapping parts of an original image with a high degree of correspondence between the original and the reproduced image.